A semiconductor wafer is manufactured typically from silicon, germanium, or gallium arsenide as having a large diameter, background to a predetermined thickness, subjected to a backside process (e.g., etching and/or polishing) according to necessity, and diced to give semiconductor chips. Recently, demands in semiconductor materials for thinner and lighter one are becoming stronger. In semiconductor wafers, it is required to reduce the thickness to 100 μm or even less, but such a thinly polished semiconductor wafer is very fragile and easily cracked. Therefore, semiconductor wafer processing has adopted a technique of supporting the wafer on a temporarily-fixing pressure-sensitive adhesive sheet (hereinafter also referred to as a “backgrinding tape”), and, after a necessary processing, stripping and recovering the backgrinding tape from the semiconductor wafer.
However, this technique has suffered from various problems. Specifically, upon affixation to the semiconductor wafer, the backgrinding tape receives force and thereby elastically deforms; the elastically deformed backgrinding tape generates force (stress) to restore, and the stress is transferred to the semiconductor wafer. However, the fragile semiconductor wafer after being polished to be thin has a too low rigidity to resist to the stress and suffers from “warping” or “sagging”, which impedes the conveyance of the semiconductor wafer. In addition, the semiconductor wafer, if “warps” too much, may break.
Such backgrinding tape generally includes an active-energy-ray-curable pressure-sensitive adhesive layer, and, after the completion of a processing step such as polishing of a semiconductor wafer, is irradiated with an active energy ray to cure the pressure-sensitive adhesive layer to have a lower adhesive strength, and is thereby stripped or peeled from the semiconductor wafer. However, the backgrinding tape may often still adhere to the semiconductor wafer by the action of atmospheric pressure, even after whose adhesive strength has been lowered through the irradiation with an active energy ray. Accordingly, to strip off the backgrinding tape, the backgrinding tape may be picked up or rubbed so as to form a peel starting point in many cases. This operation, however, often causes damage of the fragile semiconductor wafer.
A regular apparatus for stripping a backgrinding tape (backgrinding tape stripping apparatus) is configured to remove the backgrinding tape by fixing an assembly of a semiconductor wafer and the backgrinding tape after polishing (hereinafter also referred to as a “wafer after polishing”) on a vacuum chuck stage; affixing a stripping tape incorporated in the stripping apparatus to the backgrinding tape of the wafer after polishing; and stripping the backgrinding tape through peeling. The stripping tape is applied to the surface of the backgrinding tape of the wafer after polishing from an edge toward a center part thereof and is pulled up to strip off the backgrinding tape which has become unnecessary. However, it is impossible to apply such rectangular stripping tape to the round or circular backgrinding tape affixed in accordance with the round semiconductor wafer without protruding off from the edge of the backgrinding tape, and, in fact, the stripping tape may protrude off from the backgrinding tape even in a very small amount. The protruded and exposed face of the stripping tape may adhere typically to the stripping apparatus, and if the stripping tape in this state is pulled, the backgrinding tape bends together with the semiconductor wafer because of such thinness of the semiconductor wafer, and this may cause damage of the semiconductor wafer. The damage of the semiconductor wafer can be avoided by applying the stripping tape not to the edge portion but only to the core area of the backgrinding tape. In this case, however, the peel stress may not be sufficiently transferred to the backgrinding tape when the stripping tape is pulled up, and the backgrinding tape may not be removed satisfactorily.
Japanese Unexamined Patent Application Publication (JP-A) No. 2000-129223 discloses a pressure-sensitive-adhesive sheet for protecting a semiconductor wafer, including a shrinkable film, a stiff film and an energy ray-curable pressure-sensitive adhesive layer. The literature mentions that this pressure-sensitive adhesive sheet can be easily peeled off from the semiconductor wafer, by irradiating the pressure-sensitive adhesive sheet with an energy ray to allow the pressure-sensitive adhesive layer to have a lowered adhesive power and allowing the shrinkable film to shrink by a predetermined process or device, thereby causing deformation in the pressure-sensitive adhesive sheet and reducing the contact area between the semiconductor wafer and the pressure-sensitive adhesive layer. However, the present inventors selected arbitrary materials and examined similar ones to find that the pressure-sensitive adhesive sheet after heating, for example, may be folded and overlap on the semiconductor wafer surface to cause difficulties in peeling or adherend failure, typically because the contraction of a shrinkable film occurs in multiple directions.
Specifically, under present circumstances, there has been found no backgrinding tape for protecting a semiconductor wafer typically from damage during backgrinding of the semiconductor wafer, which can suppress “warping” of the semiconductor wafer even after being polished to be very thin, and, after use thereof, i.e., after the completion of backgrinding, which can be removed from the semiconductor wafer without damaging and contaminating the semiconductor wafer. In addition, there has been found no apparatus for stripping a backgrinding tape from a semiconductor wafer without damaging and contaminating the semiconductor wafer.